Right, you don't need to take the fluid cap off at all!
I'm guessing that you have bought the cheaper rewind tool, ie a "cube" - if so, then that is your main
problem.
The only tool that guarentees useful results is the one that allows you to apply pressure against the opposite side of the calliper, and that tool can be bought in Halfords etc.
Even then you might have problems rewinding the piston, but if so persevere and maybe apply the handbrake a couple of clicks to move the actuating lever up a bit.
It is clockwise that you need to turn both sides to retract them in VAG cars - the tool I mentioned only winds clockwise.